1. Field of the Invention
This invention relates to electric power-generating devices, such as wind turbines and ocean current turbines, and more particularly to an apparatus for correcting the reactive component of wind or water generated electrical power through electronic switched capacitor banks.
2. Description of the Prior Art
Wind power and tidal action in the oceans produce mechanical energy that can be captured to make electricity. Because wind and ocean energy is abundant and non-polluting, and are renewable energy sources, efforts are underway to make both wind and ocean energy economically competitive with fossil fuels and nuclear energy. Wind turbines are arrayed on land in rows where wind currents are steady, called wind farms. Tidal turbines are similar to wind turbines and would be arrayed underwater in rows, called tidal turbine farms.
Wind turbines and tidal turbines, when operating with standard induction generators, achieve output power with a lagging current with reference to the voltage. Those wind and tidal turbines with output corrected to unity will also have a lagging current with reference to the voltage at the sub-station or distribution collection point within the wind farm or tidal turbine farm. A load that tends to cause the current to become out of phase with the voltage is called a reactive load. Reactive loads are measured in Volt-Ampere Reactive units (VARs). When the current and voltage are exactly in phase this is called a unity power factor. In order to provide a power factor of unity to a utility power grid, compensation must be provided to pull the current back in phase with the voltage. This compensation must have an ability to select the number of VARs necessary to precisely compensate for the VARs introduced by the turbines and thereby bring the current and voltage in phase. The mathematical relationships between reactive power, apparent power and real power are shown by the following formulas:Apparent Power=Volts×AmperesTrue Power=Volts×Amperes×the cosine of the phase angle between volts and amps.Reactive Power=Volts×Amperes×the sine of the phase angle between volts and amps.
The relationships between apparent, true and reactive power are represented by a standard trigonometric function in FIG. 5.
Utilities and power producers have employed various methods of reactive power compensation for decades. These include correction through the generator at the generation source; correction through the use of synchronous condensers; correction through the use of contactor-switched capacitors; correction through Static VAR Compensators (SVC's); and correction through Static Synchronous Compensators (STATCOMS). These five methods are described below.
Correction Through the Generator at the Generation Source
Through their field exciters, synchronous generators have the ability to provide reactive power. Both automatic and manual controls are available to employ this reactive component to regulate line voltage, provide a fixed power factor or provide a fixed VAR (volt-ampere reactive) load for the distribution grid.
Induction generators, rectified synchronous generators and rectified permanent magnet generators employing solid state power electronics can also provide reactive power for line compensation.
When synchronous generators are corrected through their exciter, or when power electronics are employed with induction generators, the generators themselves must be capable of providing reactive power. This becomes a capital expense as the cost of the generator increases due to the increase in current requirements for the reactive load. Solid state power electronics costs also increase as the reactive load increases.
The response speed of these systems can be quite fast, on the order of ten's of milliseconds. They also offer excellent short-term capacity to help provide stability to the power distribution system.
Correction Through the Use of Synchronous Condensers
All synchronous machines (both motors and generators) are capable of generating reactive power. A synchronous condenser is simply a synchronous motor with its exciter tied to a control system to provide reactive power for a distribution grid. These are often modeled as generators with no prime mover, powered by the grid. As a motor they require real power or about 3% of the machines reactive-power rating. Therefore they are relatively inefficient, have a high capital cost and are expensive to operate and maintain. Despite these drawbacks, they are quite effective in maintaining line stability. Prior to the days of solid state power electronics they were quite popular as a means of reactive compensation for utility grids.
Correction Via Contactor-Switched Capacitors
A common method of reactive support is through mechanical contactor-switched capacitor banks. These are used on many wind farm applications where induction generators are used. Because the induction generator absorbs reactive power, the line voltage will drop as the output power from the wind farm increases (due to wind). Since capacitors generate reactive power without significant real-power losses or operating expense, they are attractive as compensation for induction generator systems.
Capacitor banks are usually stages in steps allowing for line voltage control as the power output of the wind farm changes. The use of contactors limits the switching times to no more then once per cycle, while the capacitors are limited in their re-connection capability due to requirements for discharge prior to re-connection. Contactor-switched capacitor banks are therefore quite slow in response to line voltage changes.
Non-wind farm applications employ both capacitors and inductors (called reactors) to allow for both absorption and generators of reactive power at the utility distribution level.
Capacitor banks do not have the ability to provide short-term generator support during fault or low line voltage conditions. They are also relatively expensive since most are designed for operation at medium and high voltage distribution levels.
Correction Through Static VAR Compensators (SVC's)
A Static VAR (volt-ampere reactive) Compensator or SVC is the name given to the combination of conventional capacitors and inductors with a fast solid state switch. Such systems can provide an automatic means of fast reactive support and line voltage control.
Passive Reactive components do not have the ability to provide short-term generation support during fault or low line voltage situations. They are also expensive since most are designed for operation at medium and high voltages distribution levels.
Correction Through Static Synchronous Compensators (STATCOMS)
The advent of fast, transistor based, power conversion electronics has allowed for the development of a system that synthesizes the reactive nature of both inductors and capacitors. These systems normally employ Pulse Width Modulated (PWM) Insulated Gate Bipolar Transistor (IGBT) inverters with DC links that allow not only power factor control but also short-term generation and substantial fault ride-through capability.
Like the SVC, these systems can provide very fast and effective distribution line voltage control. Like the SVC they are also relatively expensive.
In wind farm or tidal turbine farm applications wherein the turbine generators operate at a power factor of unity or less and do not have a method of power factor correction at the generator, there is a drop in the distribution line voltage as the power output of the farm increases due to increased wind speed or water current. Even in those turbines whose full power output is corrected to a unity power factor, a line voltage drop still occurs due to the impedance of the collector system transformers.
What is needed is a fast, real time control of the utility interconnected power line voltage or power factor in wind or water turbine applications where the turbine generators operate at a power factor of unity or less and do not have a method of power factor correction at the generator.
It is also desirable to provide a modular control system that is adaptable to different wind or water turbine generator types, sizes and groupings.